Sixth seed Jason Kubler made light work of 39-year-old Spaniard Fernando Verdasco in the first round of the Houston Open while Americans Steve Johnson and Denis Kudla advanced on Monday.

Kubler and Verdasco endured a 71-minute first-set tussle, which the Australian edged 9-7 in a tie-break before dominating the second frame, dishing out a bagel in 29 minutes to win 7-6 (9-7) 6-0.

World number 71 Kubler progressed to the second round where he will take on Colombia's Daniel Elahi Galan who defeated China's Zhang Zhizhen 6-2 6-3.

Johnson set up a second-round clash with American compatriot and top seed Frances Tiafoe after getting the better of Facundo Bagnis 7-6 (7-5) 1-6 6-1 in one hour and 55 minutes.

USA's Kudla also progressed with a hard-fought 6-4 4-6 6-4 victory over Australia's Aleksandar Vukic in just under two hours.

Dutchman Gijs Brouwer won 6-4 6-4 over Aleksandar Kovacevic, setting up a meeting with big-serving fourth seed John Isner.

Top seed Felix Auger-Aliassime fell to Sebastian Korda in the Estoril Open quarter-finals, while Casper Ruud was sent packing at the BMW Open. 

Auger-Aliassime won just 50 per cent of points behind his first serve as Korda claimed an impressive 6-2 6-2 success over the Canadian in one hour and 29 minutes. 

In the semi-finals Korda will take on fellow American Frances Tiafoe, who edged out Alejandro Davidovich Fokina 7-6 (7-5) 5-7 7-5 in a tight encounter. 

The other last-four clash will see Albert Ramos Vinolas take on Sebastian Baez. 

Ramos Vinolas bested Fernando Verdasco 6-2 6-2 to set up his meeting with Baez after the Argentine rallied from a set down to defeat Richard Gasquet 3-6 6-1 6-4. 

At the BMW Open, second seed Ruud fell to Botic van de Zandschulp in the quarter-finals. 

The Dutchman won their only previous encounter en route to the last eight at last year's US Open and he triumphed 7-5 6-1 in Munich. 

"Maybe the score is easier than it really was," said Van de Zandschulp. "The first set [was] really tight, I think. A great battle in the first one. I had some opportunities to break him in the beginning and didn't take them. Gladly [at] five-all I made the break and held the serve at 6-5. 

"[The] second set was a little bit easier, but I think I played really well today." 

Next up for Van de Zandschulp is Miomir Kecmanovic, who defeated Nikoloz Basilashvili 7-6 (7-5) 6-2. Holger Rune will continue his impressive run against Oscar Otte in the other semi-final after they overcame Emil Ruusuvuori and Alejandro Tabilo respectively. 

Serbian fifth seed Dusan Lajovic fought back from a set down to triumph over Benoit Paire in the Argentina Open first round while Spanish veteran Fernando Verdasco also progressed on Monday.

Lajovic, who is currently ranked 37th in the world, beat 50th-ranked Paire 4-6 6-4 7-6 (7-3) in just over two hours in Buenos Aires.

In a tight match, the Serbian got the edge in the third-set tiebreak as his first serve seldom let him down.

Former top 10 player Verdasco, now 38-years-old, defeated Bolivian Hugo Dellien 6-4 3-6 6-1 in two hours and two minutes.

Serbian eighth seed Laslo Dere got past compatriot Miomir Kecmanovic 6-4 1-6 6-1, while Argentine pair Federico Coria and Francisco Cerundolo also progressed.

Meanwhile at the Dallas Open, veteran South African Kevin Anderson proved too good for Sam Querrey 6-4 6-4.

The 35-year-old former US Open and Wimbledon finalist sent down 21 aces as he won in little over an hour.

Emerging American Brandon Nakashima defeated Australian John Millman 7-5 7-6 (7-3) in the first round.

Fifth seed Federico Delbonis was bundled out of the Cordoba Open after a shock straight-sets defeat to Carlos Taberner on Tuesday.

The 31-year-old Argentine, who is currently ranked 41st in the world, went down 6-4 6-2 to the 111th-ranked Spaniard.

Former top 10 player Fernando Verdasco was also beaten on Tuesday, going down to Argentine Sebastian Baez 7-6 (8-6) 7-5.

The 38-year-old Spaniard dropped out of the top 100 last year for the first time since 2004 after a difficult season with a long-term leg injury and COVID-19 prior to the French Open.

Baez will play Chilean third seed Cristian Garin in the second round, while Taberner will meet qualifier Alejandro Tabilo after he beat wild card Francisco Cerundolo 6-4 6-1.

Lucky loser Colombian Daniel Galan also progressed into the last 32 after a 7-5 6-3 victory over Bolivian Hugo Dellien, while sixth seed Albert Ramos-Vinolas triumphed 6-4 6-3 over Andrej Martin.

World number one Novak Djokovic will face Mats Moraing in the second round of the Belgrade Open on Tuesday.

Moraing, who is ranked 252 places below Djokovic, beat Egor Gerasimov 4-6 7-6 (9-7) 7-5 in a gruelling encounter on Monday.

Djokovic is seeking his 83rd Tour-level title as he warms up for the French Open, the main draw of which begins on May 30.

Fellow home favourite Dusan Lajovic, the highest seed to compete on day one, will meet Jeremy Chardy next after beating Ricardas Berankis in straight sets.

Eighth seed Federico Delbonis required three sets to see off Lukas Klein, while seventh seed Miomir Kecmanovic – another Serbian in the draw – was beaten 6-3 6-2 by Pablo Cuevas.

Veteran Fernando Verdasco set up a contest with Adrian Mannarino after an impressive 7-5 6-2 win over Lucas Pouille.

At the Emilia-Romagna Open in Parma, all of Monday's play had to be cancelled due to persistent rain.

Fifth seed Richard Gasquet is in action on Tuesday.